1. Field of the Invention
The present invention relates to a process for producing a phase retarder film.
2. Description of the Related Art
It is known that the viewing angle characteristic of a liquid crystal display device depends largely upon the angular dependence of the birefringence of a combination of, for example, a liquid crystal cell, a polarizing plate and a phase retarder film, used in the display device and that such a viewing angle characteristic of smaller value gives a superior result. Depending upon the constitution of the liquid crystal display device, there are cases that the phase retarder film used in the display device must have by itself a small angular dependence of birefringence, i.e. retardation.
The angular dependence of retardation of a phase retarder film is expressed by a retardation ratio R.sub.40 /R.sub.0. In this case, the retardation R.sub.40 is a value measured with the phase retarder film being tilted by 40.degree. from the horizontal condition by rotating around an axis which corresponds to the slow axis if the phase retarder film is made of a thermoplastic resin having a positive intrinsic birefringence, or to the fast axis if the phase retarder film is made of a thermoplastic resin having a negative intrinsic birefringence; the retardation R.sub.0 is a value measured with the phase retarder film not being tilted (i.e. arranged horizontally); and the measurement is made using a polarizing microscope equipped with a Senarmont compensator. (In the present invention, mere description of "retardation value" refers to R.sub.0.) As the retardation ratio of a phase retarder film is closer to 1, its angular dependence of retardation is smaller.
In order to allow a combination of a liquid crystal cell, a polarizing plate and a phase retarder film to have a small angular dependence of birefringence, it is necessary to control the R.sub.40 /R.sub.0 value of the phase retarder film depending upon the liquid crystal cell and the polarizing plate both used together with the phase retarder film.
For production of a phase retarder film having a small angular dependence of retardation, various processes are known, such as a process which comprises stretching a film whose molecules are orientated in a direction normal to the film surface [Japanese Patent Kokai (Laid-Open) No. 2-160204] and a process which comprises stretching a film produced from a molten polymer or a polymer solution in an electric field applied [Japanese Patent Kokai (Laid-Open) No. 2-285303]. These processes, however, are not satisfactory in efficiency. A process is also known which comprises adhering a shrinkable film to a resin film and subjecting the resulting laminate to thermal stretching [Japanese Patent Kokai (Laid-Open) No. 5-157911]. This process is slightly improved in the above point. As liquid crystal display devices have found wider applications in recent years, the phase retarder film used therein must have various retardation values so as to satisfy the requirements of the applications. In such a situation, the above process, however, is not fully satisfactory. Thus, there has been desired a process capable of producing a phase retarder film having a desired retardation value satisfying the intended application of a liquid crystal display device comprising said phase retarder film.
There has also been desired a process capable of producing a phase retarder film having a desired R.sub.40 /R.sub.0, in order to allow a combination of a liquid crystal cell, a polarizing plate and said phase retarder film to have a small angular dependence of birefringence.